Cobalamin dependent methionine synthesis and methyl-folate-trap in human vitamin B12 deficiency

The effects of age and sex on reference intervals for cobalamin, homocysteine, and serum and urinary methylmalonic acid in healthy adult dogs

BACKGROUND

In dogs, data on reference intervals (RIs) for cobalamin, markers of metabolism (markersB12met), age and sex effects are limited.

HYPOTHESIS/OBJECTIVES

Establish RI for serum cobalamin, homocysteine, and methylmalonic acid (sMMA) concentrations, urinary methylmalonic acid-to-creatinine ratio (uMMA:crea), and determine effects of sex and age.

METHODS

Prospective study using healthy dogs (1-10 years). Cobalamin and markersB12met were determined using chemiluminescence immunoassay (cobalamin) and liquid chromatography/tandem mass spectrometry (homocysteine, sMMA, uMMA:crea). In dogs with outlying data, changes in health, markersB12met, and onset of gastrointestinal signs were reevaluated after 9-15 months.

RESULTS

Twelve of 120 healthy dogs had abnormal uMMA:crea ratios. No other cobalamin analyte outliers were found. Outlying data re-examination (odRE) was performed in 10/12 dogs. Chronic gastrointestinal signs occurred in 64% of odRE-dogs, whereas 36% remained healthy. In total, 112 dogs (67 females, 45 males; median ages, 3.5 and 3.75 years, respectively) were included in RI analyses. Reference intervals were 178.5-851 pmol/L (cobalamin), 5.8-29.0 μmol/L (homocysteine), 45.3-159.5 μg/L (sMMA), and ≤22.4 mg/g (uMMA:crea). Only age affected cobalamin concentrations (significant decrease). Compared by sex and neuter status, intact male dogs had significantly higher uMMA:crea ratios (median, 13.5; range, 1.9-83.6 mg/g) than the other groups (median, 2.5; range, 0.7-9.7 mg/g; P < .0001). Sex-specific RI were ≤58.9 mg/g (intact male) vs ≤5.2 mg/g (females and neutered males).

CONCLUSION AND CLINICAL IMPORTANCE

Intact male dogs had significantly higher uMMA:crea ratios than the other groups. Thus, sex-specific RI are recommended for uMMA:crea. Because of the wide distribution of uMMA:crea ratios, careful interpretation in intact male dogs is advised.

Sharing vitamins: Cobamides unveil microbial interactions

Microbial communities are essential to fundamental processes on Earth. Underlying the compositions and functions of these communities are nutritional interdependencies among individual species. One class of nutrients, cobamides (the family of enzyme cofactors that includes vitamin B₁₂), is widely used for a variety of microbial metabolic functions, but these structurally diverse cofactors are synthesized by only a subset of bacteria and archaea. Advances at different scales of study-from individual isolates, to synthetic consortia, to complex communities-have led to an improved understanding of cobamide sharing. Here, we discuss how cobamides affect microbes at each of these three scales and how integrating different approaches leads to a more complete understanding of microbial interactions.

Response to exercise in older adults who take supplements of antioxidants and/or omega-3 polyunsaturated fatty acids: A systematic review

BACKGROUND

Nutrition is a key factor in determining exercise response. The aim of this review is to assess the response to exercise in older adults who take supplements of antioxidants and/or omega-3 polyunsaturated fatty acids.

METHODS

A systematic literature search was performed (June 2009- September 2019) in MEDLINE via Pubmed. The following search strategy was used with Boolean markers: ("omega-3 fatty acids" [Major] OR "antioxidants" [Major]) AND "exercise" AND "aged" [MesH]. Fourteen articles were finally included.

RESULTS

Exercise-induced free radical and inflammatory marker blood levels, but not changed the plasma total antioxidant capacity (TAC), after administration of antioxidant supplement. The oral administration of antioxidants produced null or negative effect on endothelial function, but the infusion into the brachial artery during rhythmic handgrip exercise produced a significant improvement in muscle blood flow, due to an on increase in the availability of nitric acid derived from the nitric oxide synthase. Aerobic exercise and antioxidant supplementation improved submaximal and maximal aerobic parameters, as well as mitochondrial density and mitochondria-regulated apoptotic signaling. Antioxidant supplementation, but not omega-3 PUFA, decreased pro-inflammatory marker levels and fat oxidation induced by exercise. Strength training decreased serum B₁₂ concentration but combined with omega-3 PUFA or antioxidant supplementation, B₁₂ levels were maintained. Antioxidant supplementation has protective effect after fatigue in isometric exercise but improved appendicular fat-free mass just combined with resistance exercise. Omega-3 fatty acid supplement combined with exercise increased lean mass in women, but not in men. Muscle damage induced by exercise is protected by antioxidant supplementation.

CONCLUSIONS

Older people who take antioxidant and/or omega-3 PUFA supplements showed improved exercise response, as well as lower muscle damage.

Cobalamin Deficiency Results in Increased Production of Formate Secondary to Decreased Mitochondrial Oxidation of One-Carbon Units in Rats

Background

Formate is produced in mitochondria via the catabolism of serine, glycine, dimethylglycine, and sarcosine. Formate produced by mitochondria may be incorporated into the cytosolic folate pool where it can be used for important biosynthetic reactions. Previous studies from our lab have shown that cobalamin deficiency results in increased plasma formate concentrations.

Objective

Our goal was to determine the basis for elevated formate in vitamin B-12 deficiency.

Methods

Male Sprague Dawley rats were randomly assigned to consume either a cobalamin-replete (50 μg cobalamin/kg diet) or -deficient (no added cobalamin) diet for 6 wk. Formate production was measured in vivo and in isolated liver mitochondria from a variety of one-carbon precursors. We also measured the oxidation of [3-14C]-l-serine to 14CO2 in isolated rat liver mitochondria and the expression of hepatic genes involved in one-carbon unit and formate metabolism.

Results

Cobalamin-deficient rats produce formate at a rate 55% higher than that of replete rats. Formate production from serine was increased by 60% and from dimethylglycine and sarcosine by ∼200% in liver mitochondria isolated from cobalamin-deficient rats compared with cobalamin-replete rats. There was a 26% decrease in the 14CO2 produced by mitochondria from cobalamin-deficient rats. Gene expression analysis showed that 10-formyltetrahydrofolate dehydrogenase-cytosolic (Aldh1l1) and mitochondrial (Aldh1l2) expression were decreased by 40% and 60%, respectively, compared to control, while 10-formyltetrahydrofolate synthetase, mitochondrial, monofunctional (Mthfd1l) expression was unchanged.

Conclusion

We propose that a bifurcation in mitochondrial one-carbon metabolism is a key control mechanism in determining the fate of one-carbon units, to formate or CO2. During cobalamin deficiency in rats the disposition of 10-formyl-tetrahydrofolate carbon is shifted in favor of formate production. This may represent a mechanism to generate more one-carbon units for the replenishment of the S-adenosylmethionine pool which is depleted in this condition.

Methylfolate Trap Promotes Bacterial Thymineless Death by Sulfa Drugs

The methylfolate trap, a metabolic blockage associated with anemia, neural tube defects, Alzheimer’s dementia, cardiovascular diseases, and cancer, was discovered in the 1960s, linking the metabolism of folate, vitamin B₁₂, methionine and homocysteine. However, the existence or physiological significance of this phenomenon has been unknown in bacteria, which synthesize folate de novo. Here we identify the methylfolate trap as a novel determinant of the bacterial intrinsic death by sulfonamides, antibiotics that block de novo folate synthesis. Genetic mutagenesis, chemical complementation, and metabolomic profiling revealed trap-mediated metabolic imbalances, which induced thymineless death, a phenomenon in which rapidly growing cells succumb to thymine starvation. Restriction of B₁₂ bioavailability, required for preventing trap formation, using an “antivitamin B₁₂” molecule, sensitized intracellular bacteria to sulfonamides. Since boosting the bactericidal activity of sulfonamides through methylfolate trap induction can be achieved in Gram-negative bacteria and mycobacteria, it represents a novel strategy to render these pathogens more susceptible to existing sulfonamides.

Sulfonamides were the first agents to successfully treat bacterial infections, but their use later declined due to the emergence of resistant organisms. Restoration of these drugs may be achieved through inactivation of molecular mechanisms responsible for resistance. A chemo-genomic screen first identified 50 chromosomal loci representing the whole-genome antifolate resistance determinants in Mycobacterium smegmatis. Interestingly, many determinants resembled components of the methylfolate trap, a metabolic blockage exclusively described in mammalian cells. Targeted mutagenesis, genetic and chemical complementation, followed by chemical analyses established the methylfolate trap as a novel mechanism of sulfonamide sensitivity, ubiquitously present in mycobacteria and Gram-negative bacterial pathogens. Furthermore, metabolomic analyses revealed trap-mediated interruptions in folate and related metabolic pathways. These metabolic imbalances induced thymineless death, which was reversible with exogenous thymine supplementation. Chemical restriction of vitamin B₁₂, an important molecule required for prevention of the methylfolate trap, sensitized intracellular bacteria to sulfonamides. Thus, pharmaceutical promotion of the methylfolate trap represents a novel folate antagonistic strategy to render pathogenic bacteria more susceptible to available, clinically approved sulfonamides.

How folate metabolism affects colorectal cancer development and treatment; a story of heterogeneity and pleiotropy

Folate was identified as an essential micronutrient early in the twentieth century and anti-folate chemotherapy such as 5-fluorouracil (5-FU) has been central to the medical management of solid tumours including colorectal cancer for more than five decades. In the intervening years, evidence has been gathered which shows that folate deficiency leads to many human diseases throughout the life-course. However, we still do not know all of the mechanisms behind functional folate deficiency, or indeed its rescue through supplementation with natural and particularly synthetic folates. There is growing evidence that one adverse effect of folic acid fortification programmes is an increased risk of colorectal cancer within populations. The complexity of folate-dependent, one-carbon metabolism and the heterogeneity that exists between individuals with respect to the enzymes involved in the anabolic pathways, and the catabolism of 5-FU, are explored in this review. The enzyme products of some genes such as MTHFR exert multiple and perhaps unrelated effects on many phenotypes, including cancer development. We describe this pleiotropy and the common genetic variants that affect folate metabolism; and discuss some of the studies that have investigated their potential as predictive biomarkers.

Bacterial conversion of folinic acid is required for antifolate resistance

Antifolates, which are among the first antimicrobial agents invented, inhibit cell growth by creating an intracellular state of folate deficiency. Clinical resistance to antifolates has been mainly attributed to mutations that alter structure or expression of enzymes involved in de novo folate synthesis. We identified a Mycobacterium smegmatis mutant, named FUEL (which stands for folate utilization enzyme for leucovorin), that is hypersusceptible to antifolates. Chemical complementation indicated that FUEL is unable to metabolize folinic acid (also known as leucovorin or 5-formyltetrahydrofolate), whose metabolic function remains unknown. Targeted mutagenesis, genetic complementation, and biochemical studies showed that FUEL lacks 5,10-methenyltetrahydrofolate synthase (MTHFS; also called 5-formyltetrahydrofolate cyclo-ligase; EC 6.3.3.2) activity responsible for the only ATP-dependent, irreversible conversion of folinic acid to 5,10-methenyltetrahydrofolate. In trans expression of active MTHFS proteins from bacteria or human restored both antifolate resistance and folinic acid utilization to FUEL. Absence of MTHFS resulted in marked cellular accumulation of polyglutamylated species of folinic acid. Importantly, MTHFS also affected M. smegmatis utilization of monoglutamylated 5-methyltetrahydrofolate exogenously added to the medium. Likewise, Escherichia coli mutants lacking MTHFS became susceptible to antifolates. These results indicate that folinic acid conversion by MTHFS is required for bacterial intrinsic antifolate resistance and folate homeostatic control. This novel mechanism of antimicrobial antifolate resistance might be targeted to sensitize bacterial pathogens to classical antifolates.

Cellular folate vitamer distribution during and after correction of vitamin B12 deficiency: a case for the methylfolate trap

Haematological sequellae of vitamin B12 deficiency are attributed to disturbed DNA synthesis, but vitamin B12 itself plays no role in DNA biosynthesis. A proposed explanation for this is the methylfolate trap hypothesis. This hypothesis states that B12 deficiency impairs overall folate metabolism because 5-methyltetrahydrofolate (5MTHF) becomes metabolically trapped. This trap results from the fact that 5MTHF can neither be metabolised via the methionine synthase pathway, nor can it be reconverted to its precursor, methylenetetrahydrofolate. Other manifestations of the methylfolate trap include cellular folate loss because of shorter 5MTHF polyglutamate chains and global hypomethylation. The methylfolate trap has never been demonstrated in humans. We describe a patient with B12 deficiency who was homozygous for the common methylenetetrahydrofolate reductase (MTHFR) C677T mutation. We analysed red blood cell (RBC) folate vitamers and global DNA methylation by liquid chromatography (LC) in combination with tandem mass spectrometry, and 5MTHF polyglutamate length by LC-electrochemical detection. Compared to post-B12 supplementation values, homocysteine was higher (52.9 micromol/l vs. 16.8 micromol/l), RBC folate was lower (268.92 nmol/l vs. 501.2 nmol/l), the 5MTHF fraction of RBC folate was much higher (94.5% vs. 67.4%), polyglutamate chain length was shorter (more tetra- and pentaglutamates), and global DNA methylation was 22% lower. This is the first time that virtually all features of the methylfolate trap hypothesis have been demonstrated in a human with vitamin B12 deficiency.

Folic acid

Folic acid is an essential nutrient from the B complex group of vitamins. Folate, as a cofactor, is involved in numerous intracellular reactions, and this is reflected in the various derivatives that have been isolated from biological sources. Folic acid is involved in single carbon transfer reactions and serves as a source of single carbon units in different oxidative states. The processes involved in the absorption, transport, and intracellular metabolism of this cofactor are complex. Much of folate is bound tightly to enzymes, indicating that there is not excess of this cofactor and that its cellular availability is protected as well as being strictly regulated. In animals, the liver controls the supply of folate through first pass metabolism, biliary secretion, enterohepatic recirculation, as well as through senescent erythrocyte recycling. Deficiencies of folate can occur for many reasons, including reduced intake, increased metabolism, and/or increased requirements as well as through genetic defects. The effects of folate deficiency include hyperhomocysteinemia, megaloblastic anemia, and mood disorders. Folate deficiency has also been implicated in disorders associated with neural tube defects. Supplementation of grain products such as cereals has been undertaken in several countries as a cost-effective means of reducing the prevelance of neural tube defects. Recently, common polymorphisms have been discovered in several genes associated with folate pathways that may play a role in diseases associated with folate deficiency, particularly mild folate deficiency.

Clinical, biochemical and cytokinetic parameters for distinguishing smouldering and rapidly proliferating variants of acute leukaemia

In a retrospective study, 10 patients with smouldering leukaemia (SML) were examined between 1982 and 1987. These patients typically showed the morphological criteria of acute myelogenous leukaemia (greater than 30% blasts in the bone marrow) in most cases together with a long survival time (median 16 months; 5 patients more than 22 months; 5 patients between 2.3 and 6.3 months) without the use of aggressive chemotherapy. At initial diagnosis the blast cell populations of patients with SML were characterized by significantly reduced cytosolic thymidine kinase activity (TK), thymidine-incorporation (dTR) and deoxyuridine incorporation (dUR) into DNA as well as reduced amounts of DNA-synthesizing S-phase-cells (%S) in the bone marrow (BM), compared to those patients with a rapidly proliferating acute myelogenous leukaemia (AML) and to healthy individuals. None of the SML-patients showed clinical symptoms such as night-sweat, weight-loss, hepato- and splenomegaly or lymphadenopathy at initial diagnosis. For characterization of SML vs AML we recommend the use of the biochemical parameter TK activity and the observed absence of the above-mentioned clinical symptoms. The transition to the rapidly proliferating type of AML can be recognized by an increase in the values of the biochemical and cytokinetic parameters. The blast count in the bone marrow is not suitable as a diagnostic criterion for the definition of SML vs AML or its transition to the rapidly proliferating type of AML.

Disorders of cobalamin metabolism

Recent developments in our knowledge of the biochemistry and metabolism of cobalamin have given us some insight into clinical disorders. N2O, which easily induces cobalamin deficiency, both in vivo and in vitro, has greatly contributed to the investigation of the cobalamin deficient state, especially in relation to folate and amino acid metabolism. Demonstration of the cobalamin analog in human serum and a new enzyme which requires cobalamin as a coenzyme has led to recent increased interest in this field. The disorders of cobalamin metabolism will be summarized briefly as well as those areas currently of particular interest.

[Adriamycin and autologous bone marrow transplantation. Preclinical studies in dogs (author's transl)]

We investigated the toxicity of adriamycin (ADM) in dogs with regard to autologous bone marrow transplantation (BMT). Gastrointestinal toxicity limited the dose of ADM in single administration (3.0 mg/kg) and chronic cardiotoxicity in repeated administration (9 X 1.5 mg/kg every two weeks). The recovery of hemopoiesis was complete within 25 days after single and repeated doses of ADM as indicated by the restoration of blood counts and concentration of hemopoietic precursors (CFUc) as well as proliferate activity of bone marrow. At this time bone marrow was obtained and reinfused after total body irradiation (TBI) with 10 Gy. Single and repeated doses of 1.5 mg/kg ADM did not delay recovery of blood counts after autologous BMT as compared to transplantation of untreated marrow - in contrast a single dose of 3.0 mg/kg ADM did. Our results indicate that toxicity of ADM is not influenced by autologous BMT. Previous shorter therapy with conventional doses of ADM does not preclude autologous BMT.

Vitamin B12 regulates folate metabolism by the supply of formate

Nitrous oxide (N2O) inactivates the B12 coenzyme involved in methionine synthesis and interrupts formation of the folate coenzyme (folate polyglutamate). Normal synthesis of folate polyglutamate is restored in the N2O-treated rat when folate carrying a single carbon unit is supplied at the formate level of oxidation. The activity of the enzyme, formyl synthetase, which links formate to tetrahydrofolate, is increased after exposure to nitrous oxide. Formate is normally derived from the oxidation of methyl groups, methionine being an important source. It is suggested that failure of methionine synthesis leads to a paucity of formate and in turn to inadequate formylation of tetrahydrofolate. Formyltetrahydrofolate is the required substrate for the synthesis of folate polyglutamate, and impairment of this step in turn compromises general folate metabolism.

In vitro DNA synthesis by bone marrow cells and PHA-stimulated lymphocytes. Suppression by nonradioactive thymidine of the incorporation of 3H-deoxyuridine into DNA: enhancement of incorporation when inadequate vitamin B12 or folate is corrected

Previous studies demonstrated that excess deoxyuridine (dU) added to short-term bone marrow and PHA-stimulated lymphocyte cultures, blocks the incorporation of radioactive thymidine into DNA via the salvage pathway. In the current study, we investigated the effects of added thymidine (TdR) in varying concentrations (10(-6) to 1 mumol) on the incorporation of 3H-dU into thymine-DNA, i.e. we executed 'thymidine suppression tests.' Increasing concentrations of exogenous TdR caused progressive inhibition of 3H-dU incorporation into DNA, and decreasing 3H-dU incorporation was parallelled by increasing incorporation of added 14C-TdR. These findings demonstrate reciprocity of the salvage and the de novo pathways of thymine-DNA synthesis, presumably mediated by thymidine-triphosphate (dTTP), the common end product of both pathways, via feedback inhibition. In patients with folate and/or vitamin B12 deficiency, the addition of appropriate vitamins to marrow and lymphocyte cultures enhanced the incorporation of 3H-dU into DNA. As predicted, this was not observed in normal subjects. The enhancing effect of these vitamins on in vitro incorporation of 3H-dU into DNA by deficient cell systems was similar to their correcting effect on abnormal dU suppression. These findings support the theoretical concept that the dU suppression test defines biochemical megaloblastosis due to deficiency of folate and vitamin B12.

[High-dose methotrexate/leucovorin adjuvant chemotherapy of osteogenic sarcoma: biochemical effects in DNA-synthesis of bone marrow cells (author's transl)]

DNA metabolism in bone marrow cells was measured under high-dose Methotrexate/Leucovorin adjuvant chemotherapy of a patient with primary amputation of his right leg because of osteogenic sarcoma. The biochemical data showed that there was no rescue effect of Leucovorin after 200 mg/kg Methotrexate. Corresponding to this "biochemical failure" of the rescue effect the patient died from the complications of a long and very severe bone marrow suppression. To improve the safety of this therapeutic regimen the intravenous injection and in some cases a higher dose of Leucovorin is recommended.

Interrelationships between Vitamin B12 and folic acid in myelomatosis: cobalamin coenzyme and tetrahydrofolic acid function

Cobalamin and folate metabolism was investigated in 43 patients with myelomatosis, in 8 control subjects of similar age and 22 younger controls. Plasma total cobalamin was lower in myeloma patients than in either of the control groups and methylcobalamin (Me-Cbl) was disproportionately reduced. Erythrocyte levels of total cobalamin were very similar in patients and elderly controls but were half the levels in younger controls. Erythrocyte levels of Me-Cbl were slightly higher in patients than in the dlderly controls. FIGLU excretion after L-histidine was elevated in 53% of the patients but values did not correlate with serum or erythrocyte folate or with plasma total cobalamin. FIGLU excretion decreased after DL-methionine or Me-Cbl only in patients whose FIGLU excretion was initially high. The results are discussed in the light of the 'methylfolate trap hypothesis' and suggest that some patients with myelomatosis have insufficient activity of methionine synthetase to meet the additional metabolic demand for one carbon compounds.